1. Field of the Invention
The present invention relates to a method of palletizing and depalletizing stacks of products in continuous, layer-by-layer succession and particularly to the storage and retrieval of stacks of thin, flexible sheet material. More specifically, this invention is directed to apparatus for delivering pre-formed stacks of flat and thin product onto pallets for temporary layered storage and for retrieving such products from storage and especially to apparatus for use in the palletizing and depalletizing of printing works products such as folded sheets of paper, periodicals, books, brochures and the like. Accordingly, the general objects of the present invention are to provide novel and approved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its ability, the present invention is particularly well suited for use in connection with the production of multi-page printed publications. In the manufacture of books, brochures, periodicals and other similar products, it is in many cases necessary to store large quantities of stacked printing work products on pallets pending further processing and/or to transport such products within the printing works. While it is known in the art to employ mechanical devices for loading the pallets, the unloading of individual stacks of printing work products from palletized storage has, in the past, almost universally been wholly or partly performed manually. This manual unloading is extraordinarily labor-intensive and demands heavy physical effort. Furthermore, past experience has shown that individual printing works products that are formed into stacks cannot be prevented from shifting relative to one another during the manipulation which is incident to storage and especially during the handling associated with manual unloading. As an example of the type of manual manipulation which may result in the shifting of the individual sheets which define a stack, in order to load an assembling machine, a stacker "truck" is typically utilized for transporting stacks of folded printed sheets, temporarily stored on pallets, from the in-storage area to the feeding station of the assembling machine where the operator unloads the individual stacks from the pallet by hand.
For an example of prior art apparatus for loading printing work products onto pallets in layers, reference may be had to published German Patent Application No. 31 07 495. This published application discloses a system for depositing stacks of printed newspaper or book pages, or other similar printed matter, on a pallet so that the thin flat products in question are aligned in a specific direction. The apparatus of this published application operates on the push-off principle and includes a sliding platform onto which an assembled group of the products is transferred by means of a pusher element. The sliding platform, after being loaded, moves to a position above the pallet and, after a push-off element has been lowered behind the group of products, the sliding platform is lowered to a level which is adjacent a layer of products already present and then retracted beneath the push-off element. The end result is that the grouped products are swept on to the pallet or onto a layer of products which has already been deposited on the pallet during platform retraction.
In apparatus of the type disclosed in German Application No. 31 07 495, the overall height of the platform structure results in a relatively large difference in height between the supporting surface of the platform and the top of the pallet or the top of the layer of products already present on the pallet. Due to this height difference, the products being stacked drop through a relatively large distance when being swept off of the sliding platform. This relatively long free fall unavoidably leads to the products, which have been precisely aligned on the sliding platform in a predetermined collective arrangement conforming to a defined stacking order, shifting whereby the predetermined collective arrangement is lost. This disordering is most prevalent when the products, i.e., the book pages or the like, are thin. In fact, in the case of very thin printing works products, there is considerable risk that the sliding of adjacent sheets over one another will result in the stack becoming unstable. Since it is seldom possible to perform an aligning operation once stacking has been completed, such unstability results in serious problems during destacking.
It should also be noted that, as a result of the friction that occurs between the lowermost product of a stack of product, and the supporting surface when the stack is being pushed sideways, both onto the stacking platform of the stacking apparatus and subsequently off the platform, prior art palletizing installations are incapable of fulfilling the requirement that products which are highly susceptible to damage, such as stacked portions of books or brochures, be treated with exceptional gentleness.
For a further discussion of prior art palletizing and depalletizing apparatus designed for use in a printing works, reference may be had to U.S. Pat. No. 5,135,351 which is assigned to the assignee of the present invention. The present invention constitutes an alternative to the method and apparatus of U.S. Pat. No. 5,135,351.